The change in the NHTSA's guidelines comes after Tesla Motors, the Palo Alto electric carmaker headed by CEO Elon Musk, promoted its Model S sedan as "the safest car in America," claiming the government awarded it a 5.4 star rating.

The NHTSA's new guidelines do not give crash-test ratings of more than five stars, and the agency said it would be misleading for an automaker to state they had a higher rating.

"Tesla's claim was an example of the potential confusion and inaccuracy that could be caused by incorrect use of the five-star ratings information in advertising and marketing statements," the NHTSA said in a statement. "As a result, NHTSA reviewed its advertising guidelines and made updates to ensure consumers receive accurate and consistent information."

Carmakers that diverge from the NHTSA's guidelines could receive "buyer alert" warnings from the government, or could face expulsion from the ratings program or referral to other agencies for "further, unspecified action," the Associated Press reported.

David Strickland, the NHTSA administrator, said the agency needed a way to make sure all carmakers comply with the guidelines. Speaking from the floor of the Los Angeles Auto Show, Strickland said Tesla prompted the rules revision, but added that other manufacturers had also violated the guidelines.

Earlier this week, the NHTSA began an investigation into battery fires in two of Tesla's Model S sedans. Elon Muskdefended the Model S on Tuesday, saying that his electric vehicles are the safest on the road.

On Wednesday, Tesla's website featured a revised headline that said the cars had "The highest safety rating in America," with a smaller headline that said the sedan was awarded a five-star rating in all categories.

Shares of Tesla closed 4 percent lower to $121.11 on November 20. The company's stock has dropped around 40 percent since the first fire was reported in early October.